Looks like it’s time to ditch the Speedo.

In the journal Human Reproduction Update, scientists recently presented the most complete — and mildly terrifying — data to date on the persistent decline in sperm counts among men living in the United States and Europe. What’s driving the decline remains unknown (it’s likely scientists believe tight pants could be partially to blame.

“There are things in a man’s life that will change how his testicles function,” says Allan Pacey, an expert in men’s health (read: sperm science) at the University of Sheffield in England. A poor diet, maybe, or drug use. But, Pacey says, “the single biggest risk factor for how many swimming sperm he produced each day was whether he is wearing tight pants or loose pants.”

“Skinny jeans,” he adds ruefully, “I guess they’re the fashion.”

But how does it work?

See also: Sperm Radar Might Illuminate Male Infertility

Borat mankini banana hammock
This is the opposite of what you should wear if you're a man who likes sex and cares about your swimmers.

Basically, the size of a man’s testicles are pre-programmed in utero. And unfortunately, size does matter here. Just like any manufacturing plant, the more, er, equipment, there is, the higher the output. So the bigger a dude’s testicles are, the more sperm they’ll be able to produce.

“There is a maximum amount of sperm that he can produce each day, based on the size of his testicles,” Pacey says. But most men never hit that. “Everything he does decreases the optimization of that count.” And that includes strangling your ballsack with super-tight clothing.

That’s because the tighter the clothes, the more liable the testicles are move up toward his body’s warm core. But because the scrotum has to be relatively cool to produce healthy sperm, any kind of intense heat, whether it’s from your laptop or by your itsy bitsy mankini can alter sperm’s bendiness. And when sperm don’t wriggle right, they’ll never make it to the egg.

Fortunately, the sperm-making machine is working day and night, so the negative effects of Speedos can be reversed. If you’re a skinny jeans wearer, it should take only three months of wearing appropriately-sized pants to reverse your fortunes. That’s because three months is about how long it takes to make a fresh batch of sperm from start to finish.

Of course, sperm count matters more to men trying to conceive. Pacey recommends anyone with trouble conceiving reach out to their doctor to discuss potential risk factors, including body-hugging clothes. He says they should talk to a doctor before making any lifestyle (or outfit) changes so that they and their doctor can monitor progress together.

As for guys not in the baby-making mood, don’t treat tight jeans as a contraceptive. A mankini may reduce your sperm count, but it doesn’t eliminate the possibility of pregnancy altogether.

Photos via Giphy